1. Field of the Invention
The present invention relates to an optical recording medium and, more specifically, to a record blank suitable for recording information with a laser beam, in particular, a semiconductor laser beam, and to an information record allowing optical reproduction of information recorded by a laser beam.
2. Description of the Related Art
Generally, information can be recorded on an optical recording medium, such as a series of optical discs or optical cards, by forming an optically detectable minute pits (for example, of approximately 1 .mu.m) arranged in such a manner as to define a spiral, concentric or linear track on a recording layer formed on a substrate of the recording medium, thereby making it possible to store information at high-density.
As described, for example, in "Review and Analysis of Optical Recording Media" of Optical Engineering, Vol.15, No.2, March-April 1976, pp99.about., a known optical recording method consists in applying a light beam, such as a laser beam, to the recording layer of an optical recording medium so as to generate deformation or pits on the recording layer. In other known methods, the application of such a light beam causes the generation of bubbles, changes in phase, discoloration, discolorization or the like.
Various materials have been proposed for the recording layer of such an optical recording medium. Examples of such materials include inorganic materials, such as a metal film consisting of an aluminum deposit film or the like, a bithmuth film, a tellurium oxide film, or a chalcogenite-type non-crystalline glass film. Generally, such thin films are sensitive to light having a wavelength of approximately 350 to 800 nm and exhibit high reflectance to laser beams. One disadvantage of these films is that they provide a rather poor laser-beam-utilization factor. Further, such an inorganic material thin film is usually formed as a recording layer by sputtering or the like. Such a film formation method, however, requires a production line equipped with a vacuum system, so that these inorganic material thin films have higher production costs.
In view of the above problems, optical recording mediums using organic coloring matter whose optical properties can be changed by the energy of light having a relatively long wavelength (for example, 780 nm or more) are being carefully studied. Optical recording mediums using such organic coloring matter are effective in that they allow the formation of pits by a semiconductor laser having an oscillation wavelength of around 780 nm or 830 nm. Further, organic coloring matter allows film formation by wet coating and can be easily treated, making it possible to easily mass-produce recording mediums on equipment costing less.
In this context, for example, in Japanese Patent Laid-Open No. 1 -26879, an optical recording medium which uses a polymethine dye for the recording layer and which allows high S/N recording by a semiconductor laser and exhibits satisfactory heat stability is shown.
As stated above, employment of an organic coloring matter for the recording layer helps to reduce production costs of optical recording mediums because it allows film formation by wet coating. Such an organic coloring matter, however, must exhibit a maximum absorptivity in a range near the wavelength of the recording light beam, excellent heat stability, and high solubility in solvent. In a case where an organic coloring matter having poor solubility in solvent is used, a slight change in the concentration of the solution to be applied would likely cause solid particles, e.g., minute crystals of the organic coloring matter, in the solution. In that case, a wet coating process would lead to mixing the solid particles in the solution for preparing recording layers to the recording layer, resulting in low-quality optical recording mediums exhibiting a high noise level at the time of reproduction.